This disclosure relates to a process for the production of branched polycarbonates.
Aromatic polycarbonates are used in a variety of applications due to their excellent mechanical and physical properties including, among others, impact and heat resistance, strength and transparency. There are three general processes known for the commercial manufacture of aromatic polycarbonates, which are illustrated in FIGS. 1-3. The conventional interfacial process, as shown in FIG. 1, and the phosgene-based melt process, as shown in FIG. 2, start with the reaction of phosgene with carbon monoxide. The third general process, a xe2x80x9cnon-phosgenexe2x80x9d melt process as shown in FIG. 3, was developed to eliminate the use of highly toxic phosgene in the process flow. Of these general methods, the xe2x80x9cnon-phosgenexe2x80x9d melt process shown (also referred to as the melt transesterification process) is preferred since it prepares polycarbonates less expensively and with better optical properties than the interfacial process and avoids the use of highly toxic phosgene.
Both types of melt processes (FIGS. 2, 3) make use of a diarylcarbonate, such as diphenylcarbonate as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction scheme shown in FIG. 4. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers.
In certain applications, it is desirable to use a branched polycarbonate with high melt strength. For example, blow molding of bottles and extrusion of sheet products from polycarbonate requires the polycarbonate to have high melt strength. Moreover, branched polycarbonate resins can be used in extrusion processes for the production of profiles, solid sheets, multi-wall sheets or corrugated sheets.
For example, DE 1570533 to Fritz et al. discloses a method for making branched polycarbonate by adding 0.25 to 1.5 mole percentage (with respect to the bisphenol) of a phenol with a functionality higher than 2. The example of DE-1570533 teaches the use of 4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-2-heptene or trimeric isopropenyl phenol. It should be noted, however, that the use of branching agents having benzylic hydrogens and double bonds like trimeric isopropenyl phenol lead to the development of discoloration in melt processed polycarbonates.
DE 19727709 to Bunzar et al. discloses a method for making branched melt polycarbonate using 3 to 6 functional aliphatic alcohols. In particular, the examples of DE 19727709 teach a method of polymerizing the branching agents pentaerythritol or dipentaerythritol directly together with the monomers Bisphenol A and diphenyl carbonate. The use of aliphatic alcoholic monomers instead of aromatic phenolic monomers leads to reductions in reaction rates during melt polymerization, lower thermal stability and discoloration in the resulting polymer. In addition, directly polymerizing branching agents together with aromatic dihydroxy compound and diaryl carbonate monomers typically requires complicated separation units to be used in continuous melt reactor systems if the phenol byproduct and/or diaryl carbonate are to be recycled after their removal from the reactors. Further, adding a branching agent to the first reactor or oligomization section of a continuous reactor system requires long operating times for the transition from the stable operation for producing linear polycarbonate to that for producing branched polycarbonate having the desired target properties. This long transition time also requires the production of large quantities of a transition material having variable levels of branching.
U.S. Pat. No. 5,693,722 to Priddy Jr. et al. discloses a method for making a branched polycarbonate by first synthesizing a polycyclic-oligocarbonate and then melt-mixing the polycyclic-oligocarbonate together with a polycarbonate resin. This multi-step process requires separate and complicated reaction systems for preparing each of the two reactants, the polycyclic oligocarbonate and the polycarbonate, and for the subsequent reaction. It should be noted that polycyclic-oligocarbonates are prepared from chloroformates in solution, and as a result the melt polycarbonates contain residual solvents and chlorinated compounds, both of which characteristics are undesirable in terms of handling, environmental, and product quality standpoints.
EP 708130 to King et al. discloses a method for producing a blow moldable polycarbonate by first producing a polycarbonate preform by a melt transesterification process which contains a polyfunctional branching agent. The polycarbonate preform is then melt equilibrated with a second polycarbonate to produce a blow moldable grade of polycarbonate. This multi-step process requires separate and complicated reaction systems for preparing each of the two reactants, the polycarbonate preform and the polycarbonate, and for their reaction together.
Accordingly, there remains a need in the art for an improved process for the production of branched polycarbonates having high melting strengths in a continuous melt reaction system.
The above mentioned drawbacks and disadvantages are overcome or alleviated by a process for the production of branched aromatic polycarbonates that includes reacting a diarylcarbonate and a polyhydric alcohol in the presence of an alkaline catalyst to produce a polycarbonate oligomer; adding a branching agent to the polycarbonate oligomer, wherein the branching agent has the formula (I):
AGyxe2x80x83xe2x80x83(I)
wherein A is a C1-20 polymethylene, C2-20 alkylene or alkylidene, C5-36 cycloalkylene or cycloalkylidene, C6-36 arylene or alkylarylene, or C6-36 arylalkylene, wherein G is a monovalent C6-C30 hydrocarbon having at least one hydroxyl group bonded directly to an aromatic or cycloaliphatic ring and y is an integer greater than 2, and wherein each G may be the same or different; and producing a branched aromatic polycarbonate having a melt index ratio greater than that of an aromatic polycarbonate produced from the polycarbonate oligomer without the addition of the branching agent.
In one embodiment, the branching agent is added in combination with a carbonic acid diester to a polycarbonate oligomer produced during the polymerization of polycarbonate. The carbonic acid diester comprises a compound selected from the group consisting of compounds having formula (IV): 
wherein R1 and R2 may be the same or different and are selected from the group consisting of phenyl, C6-20 aryl, C6-20 arylalkyl groups, and wherein R1 and R2 may optionally be substituted with activating groups.
These and other features will be apparent from the following brief description of the drawings, detailed description, and attached drawings.